1. Field of the Invention
The present invention relates generally to telecommunications and, more particularly, to wireless telecommunication systems.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Although the first mobile telephone system for public use was developed in 1946, and improved in 1965, modern wireless technology was introduced in 1970 as the Advanced Mobile Phone Service (AMPS), which is the American analog cellular standard. Despite this early development, the first commercial cellular system began operating in Chicago in 1983, thus giving rise to one of the fastest growing consumer technologies in history. Indeed, so many people subscribed to cellular service by the mid-1990s that the critical problem for cellular carriers became that of capacity. Accordingly, cellular providers had to develop ways to derive more capacity. The most extreme and costly method to increase capacity involved reducing cell sizes and introducing additional base stations. However, in many large metropolitan areas, it became increasingly difficult and costly to obtain permits to erect base stations and antennas. Accordingly, cellular providers desired a solution for increasing system capacity without requiring more base stations. One proposed solution involved the use of digital technology.
The first all digital systems, Personal Communication Services (PCS), were introduced in the United States in the mid-1990s. PCS is referred to as the second generation wireless service, with the first generation mobile telephone service being the analog service mentioned above. Various digital wireless technologies were developed, including Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA), and Global System for Mobile Communications (GSM). Because the advent of digital wireless communications greatly increased the capacity of wireless networks, cellular providers had more capacity to sell to eager subscribers.
As described above, there are various digital technologies. In FDMA, every phone call is assigned to a unique frequency. This technology worked well with a sMAIl number of users, but as the number of users grew, there were simply not enough frequencies for each user. One method of overcoming this limitation was the technology known as TDMA. In TDMA, one frequency is further divided into several time slots. Individual phone calls are then assigned to each time slot. In this way, TDMA allows multiple users to share one frequency, thus increasing the number of users at any one time. Unfortunately, TDMA can not provide enough time slots to satisfy the growing demand.
CDMA is one technique that addresses this problem. In a CDMA system, phone calls are no longer divided by frequency or time slot. Rather, all phone calls are transmitted at the same time and at the same frequency. While this method may appear chaotic, each individual phone or mobile device is able to recognize its call by a unique code assigned to that call. This unique code allows many users to share a single frequency while permitting a greater degree of privacy and security than FDMA or TDMA.
As described above, CDMA systems permit many users to share a single frequency. One potential problem of this type of frequency sharing is multi-access interference. Multi-access interference (“MAI”) occurs when a particular signal (user A's phone call, for example) is distorted by other signals sharing the same frequency. Typically, the other signals are other phone calls sharing the same frequency. MAI typically manifests itself as noise, which can distort or mask the transmitted signal. This noise can adversely affect call clarity and limit the number of individual phone calls that can share a single frequency.
Because lower power signals generate less MAI, one technique for reducing this MAI (and thus reducing the noise) is to reduce signal power. Unfortunately, reducing signal power is typically difficult because transmission quality is related to signal power, and all other factors being equal, a signal transmitted at a higher power will arrive at a receiver with fewer errors than a signal transmitted at a lower power. More importantly, a signal transmitted with too little power may be overshadowed by noise on the frequency. If parts of user A's phone call can not readily be distinguished from noise, there can be errors in the phone call. These errors in the phone call are typically measured in frame error rate (“FER”). The FER is the ratio of the data (measured in terms of number of frames where a frame consists of a pre-specified number of bits) transmitted with errors to the total data transmitted. High FERs can result in problems with call clarity such as phone call gaps or dropped calls. Because mobile phone users are typically concerned about call clarity, the providers of mobile phone services are hesitant to reduce signal power at the expense of call clarity.
That being said, mobile phone providers are still very interested in techniques for reducing signal power without sacrificing call clarity because lower power signals create less MAI and may permit more mobile phones to share a single frequency. One method of reducing the power of the other signals without sacrificing call clarity is through signal encoding and decoding. Signal encoding is the altering of the characteristics of a signal to make the signal more suitable for transmission. For example, a signal encoder in a transmitter may add error-correction bits to a signal. These error-correction bits are then used by a signal decoder in a receiver to correct errors that may have developed in the encoded signal during transmission. By allowing the receiver to correct errors that might have developed during transmission, the signal encoder and decoder may permit a transmitter to transmit the signal at a lower power without an increase in the FER. In other words, signal encoding and decoding can offset the effects of transmitting at a lower power. Thus, the better the signal encoding and decoding scheme, the lower the power typically needed to transmit the signal.
One encoding/decoding technique, known as turbo coding, has a greater error correction capability than many previously used codes. In fact, the introduction of turbo codes in 1993 was considered as one of the most exciting and important developments in digital communications in many years. By using turbo codes, data can be transmitted within 0.7 dB of the signal to noise ratio (SNR) as dictated by the Shannon limit, which gives the minimum theoretical SNR for error-free transmission. This high level of error correction permits transmitters to transmit signals at a lower power without increasing the FER. For this reason, turbo coding is a primary candidate for adoption in the next generation of cell phone (known as third generation or “3G” cell phones).
The channel is another factor that can increase the signal power needed to maintain an acceptable PER. The channel includes the net effect of environmental factors, such as the weather, the Earth's magnetic fields, terrain variations, structures, or vehicles, on a signal. Mathematically, after removing the high frequency carrier, it can be represented as a complex number multiplied by the original signal plus noise for a frequency non-selective channel where there is only one copy of the transmitted signal that is received. In order to convert the signal back into a voice or other useful data, a receiver may attempt to compensate for these environmental effects by estimating the channel. If a receiver were able to make a perfect estimate of the channel, the receiver would be able to convert the received signal back into an exact copy of the transmitted signal (assuming no other disturbances like noise or multi-path disturbances). Unfortunately, it is virtually impossible to perfectly estimate the channel. Thus, the receiver is typically not able to compensate for all of the net environmental effects and some additional distortion is introduced at the receiver. Typically, a pilot signal is used to estimate the channel, where a pilot signal includes known symbols that the receiver can be use to calculate an estimate of the channel. The pilot signal typically occupies the same frequency as the user's data signal and, hence, contributes to the MAI in the same way.
As with the noise introduced by MAI, one typical way to compensate for the distortion generated by the imperfect channel estimate is to increase the signal and/or pilot power. As described above, however, increasing the signal and/or pilot power is not preferred because it increases MAI, which may affect call clarity and reduce the number of phone calls that can share the same frequency. Thus, a technique for improving the quality of the channel estimation at lower pilot signal power or lower user's data signal power is desirable because call clarity could be maintained and the total signal power (data signal power plus pilot signal power) used to compensate for imperfect channel estimation could be lowered.